Thick single crystal diamond layer method for making it and gemstones produced from the layer

ABSTRACT

A layer of single crystal CVD diamond of high quality having a thickness greater than 2 mm. Also provided is a method of producing such a CVD diamond layer.

This is a continuation application of U.S. application Ser. No.11/486,421, filed Jul. 14, 2006, which is a continuation application ofU.S. application Ser. No. 10/739,014, filed Dec. 19, 2003, which is acontinuation of U.S. application Ser. No. 10/297,591, filed Dec. 13,2002, abandoned, which is a 371 of PCT/IB01/01040 filed on Jun. 14,2001.

BACKGROUND OF THE INVENTION

This invention relates to diamond and more particularly to diamondproduced by chemical vapour deposition (hereinafter referred to as CVD).

Methods of depositing materials such as diamond on a substrate by CVDare now well established and have been described extensively in thepatent and other literature. Where diamond is being deposited on asubstrate, the method generally involves providing a gas mixture which,on dissociation, can provide hydrogen or a halogen (e.g. F,Cl) in atomicform and C or carbon-containing radicals and other reactive species,e.g. CH_(x), CF_(x) wherein x can be 1 to 4. In addition,oxygen-containing sources may be present, as may sources for nitrogen,and for boron. In many processes inert gases such as helium, neon orargon are also present. Thus, a typical source gas mixture will containhydrocarbons C_(x)H_(y) wherein x and y can each be 1 to 10 orhalocarbons C_(x)H_(y)Hal_(z) (Hal=halogen) wherein x and z can each be1 to 10 and y can be 0 to 10 and optionally one or more of thefollowing: CO_(x), wherein x can be 0.5 to 2, O₂, H₂, N₂, NH₃, B₂H₆ andan inert gas. Each gas may be present in its natural isotopic ratio, orthe relative isotopic ratios may be artificially controlled; for examplehydrogen may be present as deuterium or tritium, and carbon may bepresent as ¹²C or ¹³C. Dissociation of the source gas mixture is broughtabout by an energy source such as microwaves, RF energy, a flame, a hotfilament, or a jet based technique and the reactive gas species soproduced are allowed to deposit onto a substrate and form diamond.

CVD diamond may be produced on a variety of substrates. Depending on thenature of the substrate and details of the process chemistry,polycrystalline or single crystal CVD diamond may be produced. Theproduction of homoepitaxial CVD diamond layers has been reported in theliterature. Prior art has generally concerned itself with the thermal,optical and mechanical properties of CVD diamond.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided a layerof single crystal CVD diamond of high quality having a thickness of atleast 2 mm, and preferably a thickness of greater than 2.5 mm, and morepreferably a thickness of greater than 3 mm.

The high quality of the diamond may be characterised by one or more ofthe following characteristics. These characteristics are observable inthe majority volume of the layer or stone or in the {100} growth sectorwhen present and discernible:

-   1) A high charge collection distance of at least 100 μm, preferably    at least 150 μm, and more preferably at least 400 μm, all collection    distances being measured at an applied field of 1 V/μm and at 300 K    (or 20° C., which for the purposes of this invention is considered    equivalent). In high quality type IIa natural diamond charge    collection distances are reported to be substantially less than 100    μm, and more typically about 40 μm at an applied field of 1 V/μm.-   2) A high value for the product of the average carrier mobility and    lifetime μτ, such that it exceeds 1.0×10⁻⁶-cm²/V, and preferably    exceeds 1.5×10⁻⁶ cm²N, and more preferably exceeds 4×10⁻⁶ cm²N, all    measurements at 300 K.-   3) In the off state, a resistivity measured at 300 K greater than    10¹² Ωcm at an applied field of 50 V/μm, and preferably greater than    2×10¹³ Ωcm, and more preferably greater than 5×10¹⁴ Ωcm.    -   In a wide band gap device such as one fabricated from diamond,        the number of free charge carriers present under equilibrium        conditions is extremely small and dominated by the contribution        from lattice defects and impurities, such a device is said to be        in the “off state”. The device can be put into the “on state” by        the additional excitation of charge carriers by means such as        optical excitation (primarily using optical energies near or        greater than the band gap) or by charged particle excitation        (e.g. alpha or beta particles). In the on state the free carrier        density exceeds the equilibrium level and when the excitation        source is removed the device will revert to the off state.-   4) An electron mobility (μ_(e)) measured at 300 K greater than 2400    cm²V⁻¹s⁻¹, and preferably greater than 3000 cm²V⁻¹s⁻¹, and more    preferably greater than 4000 cm²V⁻¹s⁻¹. In high quality type IIa    natural diamond electron mobilities are reported typically to be    1800 cm²V⁻¹s⁻¹ at 300 K with exceptional values reported up to 2200    cm²V⁻¹s⁻¹.-   5) A hole mobility (μ_(h)) measured at 300 K greater than 2100    cm²V⁻¹s⁻¹, and preferably greater than 2500 cm²V⁻¹s⁻¹, and more    preferably greater than 3000 cm²V⁻¹s⁻¹. In high quality type IIa    natural diamond hole mobilities are reported to be typically 1200    cm²V⁻¹s⁻¹ at 300 K with exceptional values reported up to 1900    cm²V⁻¹s⁻¹.

It will be noted from the above that the diamond of the invention haselectronic characteristics which are significantly superior to thosepresent in natural high quality diamond. This is surprising and providesthe diamond with properties which are useful, for example, forelectronic applications where thick layers are required and also for theeconomic production of thinner layers for other electronic devices.There is benefit in synthesising a single thick layer and processing itinto multiple thinner layers because of the reduced overheads in termsof substrates and synthesis preparation.

The diamond of the invention is also suitable for use as diamond anvilsin high pressure experiments and manufacture where the low defectdensity of the diamond makes it much stronger than natural diamond andable to operate under more extreme conditions of temperature andpressure.

The diamond of the invention has a thickness suitable to allow for theproduction through cutting, for example, of one or more gemstonestherefrom.

In addition to the characteristics described above, the diamond layer ofthe invention may have one or more of the following characteristics:

-   1) A level of any single impurity of not greater than 5 ppm and a    total impurity content of not greater than 10 ppm. Preferably the    level of any impurity is not greater than 0.5 to 1 ppm and the total    impurity content is not greater than 2 to 5 ppm. Impurity    concentrations can be measured by secondary ion mass spectroscopy    (SIMS), glow discharge mass spectroscopy (GDMS) or combustion mass    spectroscopy (CMS), electron paramagnetic resonance (EPR) and IR    (infrared) absorption, and in addition for single substitutional    nitrogen by optical absorption measurements at 270 nm (calibrated    against standard values obtained from samples destructively analysed    by combustion analysis). In the above, “impurity” excludes hydrogen    and its isotopic forms.-   2) A cathodoluminescence (CL) line at 575 nm which is low or absent,    and associated photoluminescence (PL), measured at 77 K under 514 nm    Ar ion laser excitation (nominally 300 mW incident beam) which has a    peak height< 1/25 and preferably < 1/300 and more preferably <    1/1000 of the diamond Raman peak at 1332 cm⁻¹. These bands are    related to nitrogen/vacancy defects and their presence indicates the    presence of nitrogen in the film. Due to the possible presence of    competing quenching mechanisms, the normalised intensity of the 575    nm line is not a quantitative measure of nitrogen nor is its absence    a definitive indication of the absence of nitrogen in the film. CL    is the luminescence resulting from excitation by electron beam at a    typical beam energy of 10 to 40 keV which penetrates about 10    microns into the sample surface. Photoluminescence is more generally    excited through the sample volume.-   3)    -   (i) Strong free exciton (FE) emission in the cathodoluminescence        spectrum collected at 77 K.        -   Free exciton emission is quenched by point defects and            structural defects such as dislocations. The presence of            strong free exciton emission in the cathodoluminescence            spectrum indicates the substantial absence of dislocations            and impurities. The link between low defect and impurity            densities and high FE emission intensity has previously been            reported for individual crystals in polycrystalline CVD            diamond synthesis.    -   (ii) Strong free exciton emission in the room temperature        UV-excited (ultra violet-excited) photoluminescence spectrum.        -   Free exciton emission can also be excited by above-bandgap            radiation, for example by 193 nm radiation from an ArF            excimer laser. The presence of strong free exciton emission            in the photoluminescence spectrum excited in this way            indicates the substantial absence of dislocations and            impurities. The strength of free exciton emission excited by            193 nm ArF excimer laser at room temperature is such that            the quantum yield for free exciton emission is at least            10⁻⁵.-   4) In electron paramagnetic resonance (EPR), a single substitutional    nitrogen centre [N—C]⁰ at a concentration<100 ppb and typically <40    ppb and more typically <20 ppb indicating low levels of nitrogen.-   5) In EPR, a spin density<1×10¹⁷ cm⁻³ and more typically <5×10¹⁶    cm⁻³ at g=2.0028. In single crystal diamond this line is related to    lattice defect concentrations and is typically large in natural type    IIa diamond, in CVD diamond plastically deformed through    indentation, and in poor quality homoepitaxial diamond.-   6) Excellent optical properties having a UV/Visible and IR    (infra-red) transparency close to the theoretical maximum for    diamond and, more particularly, low or absent single substitutional    nitrogen absorption at 270 nm in the UV, and low or absent C-H    stretch absorption in the spectral range 2500 to 3400 cm⁻¹ in the    IR.

The characteristics described above will be observable in the majorityvolume of the layer or stone. There may be portions of the volume,generally less than 10 percent by volume, where the particularcharacteristic is not observable.

The invention provides, according to another aspect, a synthetic diamondin the form of a gemstone produced from a layer of the type describedabove.

The novel thick single crystal CVD diamond layer of the invention may bemade by a method which forms yet another aspect of the invention. Thismethod includes the steps of providing a diamond substrate having asurface substantially free of crystal defects, providing a source gas,dissociating the source gas and allowing homoepitaxial diamond growth onthe surface of low defect level to occur in an atmosphere which containsless than 300 parts per billion nitrogen. It has been found that thicksingle crystal CVD diamond layers of high quality may be produced if adiamond substrate having a surface substantially free of crystal defectsis used and if the homoepitaxial growth occurs in an atmosphere whichcontains less than 300 parts per billion molecular nitrogen.

The importance of achieving a substrate surface substantially free ofsurface defects on which to synthesise thick layers is that such defectscause dislocations and associated defects in the overgrown CVD layer,and that once present these dislocation structures cannot simplyterminate in the layer but generally multiply and expand, resulting instress, defects and cracks as the layer is grown thicker. Nitrogen inthe process, even at very low levels, plays a role in controlling themorphology of the growing surface, resulting in stepped growth whichagain causes dislocated and defective growth as the layer increases inthickness.

The invention further provides a CVD diamond produced from a singlecrystal CVD layer described above polished in the form of a gemstonecharacterised by having three orthogonal dimensions greater than 2 mm,and preferably greater than 2.5 mm, and more preferably greater than 3.0mm, where at least one axis lies either along the <100> crystaldirection or along the principle symmetry axis of the stone. The diamondwill be of high quality and may have one or more of the characteristicsidentified above.

DESCRIPTION OF EMBODIMENTS

The single crystal CVD diamond layer of the invention has a thickness ofat least 2 mm and is of high quality, and particularly is of highcrystalline perfection and purity. This is evidenced by the diamondhaving one or more of the characteristics identified above.

The collection distance may be determined by methods known in the art.The collection distances referred to in this specification weredetermined by the following procedure:

-   1) Ohmic spot contacts are placed on either side of the layer under    test. This layer is typically 300-700 μm thick and 5-10 mm square,    allowing spot contacts of 2-6 mm diameter. Formation of ohmic    contacts (rather than contacts showing diode behaviour) is important    for a reliable measurement. This can be achieved in several ways but    typically the procedure is as follows:    -   i) the surface of the diamond is oxygen terminated, using for        example, an oxygen plasma ash, minimising the surface electrical        conduction (reducing the ‘dark current’ of the device);    -   ii) a metallisation consisting of first a carbide former (e.g.        Ti, Cr) and then a thicker layer of protective material,        typically Au (to which a wire bond can be made), is deposited        onto the diamond by sputtering, evaporation or similar method.        The contact is then typically annealed between 400-600° C. for        about an hour.-   2) Wire bonds to the contacts are made, and the diamond connected in    a circuit, with a bias voltage of typically 2-10 kV/cm. The ‘dark    current’ or leakage current is characterised, and in a good sample    should be less than 5 nA, and more typically less than 100 pA at 2.5    kV/cm.-   3) The collection distance measurement is made by exposing the    sample to beta radiation, with a Si trigger detector on the exit    face to a) indicate that an event has occurred, and b) ensure that    the beta particle was not stopped within the diamond film which    would result in a much larger number of charge carriers being    formed. The signal from the diamond is then read by a high gain    charge amplifier, and, based on the known formation rate of charge    carriers of about 36 electron/hole pairs per linear μm traversed by    the beta particle, the collection distance can be calculated from    the charge measured by the equation:    CCD=CCE×t    -   where t=sample thickness    -   CCE=charge collection efficiency=charge collected/total charge        generated.    -   CCD=charge collection distance.    -   It is clear that the charge collection distance measured is        limited to the sample thickness; this is expressed in the Hecht        relationship given later.-   4) For completeness, the collection distance is measured for a range    of values of applied bias voltage, both forward and reverse, and the    characteristic collection distance quoted at bias voltages of 10    kV/cm only for samples which show a well behaved linear behaviour    for bias voltages up to 10 kV/cm bias. In addition, the entire    measurement procedure is repeated several times to ensure    repeatability of behaviour, as values measured on poorer samples can    degrade with time and treatment history.-   5) A further issue in measurement of the collection distance is    whether the material is in the pumped or unpumped state. ‘Pumping’    (also called ‘priming’) the material comprises of exposing it to    certain types of radiation (beta, gamma etc.) for a sufficient    period, when the collection distance measured can rise, typically by    a factor of 1.6 in polycrystalline CVD diamond although this can    vary. The effect of priming is generally lower in high purity single    crystal diamond; priming by factors of 1.05-1.2 is common with no    measurable priming in some samples. De-pumping can be achieved by    exposing to sufficiently strong white light or light of selected    wavelengths, and the process is believed to be wholly reversible.    The collection distances referred to in this specification are all    in the unpumped state whatever the final application of the    material. In certain applications (e.g. high energy particle physics    experiments), the increase in collection distance associated with    pumping can be used beneficially to enhance the detectability of    individual events, by shielding the detector from any de-pumping    radiation. In other applications, the instability in device gain    associated with pumping is severely deleterious.

The single crystal CVD diamond of the invention may, in one form of theinvention, have, in the off state, a high resistivity at high appliedfields and more particularly a resistivity R₁ exceeding 1×10¹² Ωcm, andpreferably exceeding 2×10¹³ Ωcm and more preferably exceeding 5×10¹⁴Ωcm, at an applied field of 50 V/μm measured at 300 K. Suchresistivities at such high applied fields are indicative of the purityof the diamond and the substantial absence of impurities and defects.Material of lower purity or crystal perfection can exhibit highresistivity at lower applied fields, e.g. <30 V/μm, but shows breakdownbehaviour with rapidly rising leakage currents at applied fields greaterthan 30 V/μm and generally by 45 V/μm. The resistivity can be determinedfrom a measurement of the leakage (dark) current by methods known in theart. A sample under test is prepared as a plate of uniform thickness,cleaned using standard diamond cleaning techniques in order to acceptsuitable contacts (evaporated, sputtered or doped diamond) to whichexternal connections can be made to the voltage supply, and thenpartially or wholly encapsulated to minimise risk of flash-over. It isimportant to ensure that the encapsulation does not add significantly tothe leakage current measured. Typical sample sizes are 0.01-0.5 mm thickby 3×3 mm-50×50 mm laterally, but smaller or larger sizes may also beused.

The single crystal CVD diamond of the invention may have a μτ productgreater than 1.0×10⁻⁶ cm²/V, preferably a μτ product of greater than1.5×10⁻⁶ cm²/V and more preferably a μτ product greater than 4.0×10⁻⁶cm²/V. The μτ product is related to the charge collection distance usingthe following equation:μτE=CCD(cm²/Vs)×(s)×(V/cm)=cm

-   -   where E=applied field

The single crystal CVD diamond of the invention, particularly in itspreferred form, has a high μτ product which translates into a highcharge collection distance.

When an electric field is applied to a sample using electrodes it ispossible to separate the electron-hole pairs generated by photonirradiation of the sample. The holes drift toward the cathode and theelectrons toward the anode. Light with a short wavelength (UV light) anda photon energy above the bandgap of the diamond has a very smallpenetration depth into the diamond and by using this type of light it ispossible to identify the contribution of one type of carrier onlydependent on which electrode is illuminated.

The μτ product referred to in this specification is measured in thefollowing way:

-   -   (i) A sample of diamond is prepared as a plate in excess of ≈100        μm thick.    -   (ii) Ti semi-transparent contacts are sputtered onto both sides        of the diamond plate and then patterned using standard        photolithography techniques. This process forms suitable        contacts.    -   (iii) A 10 μs pulse of monochromatic Xe light (wavelength 218        nm) is used to excite carriers, with the photocurrent generated        being measured in an external circuit. The pulse length of 10 μs        is far longer than other processes such as the transit time and        the carrier lifetime and the system can be considered to be in        equilibrium at all times during the pulse. The penetration of        light into the diamond at this wavelength is only a few microns.        Relatively low light intensity is used (about 0.1 W/cm²), so        that the total number of electron hole pairs generated by the        light pulse is relatively low and the internal field is then        reasonably approximated by the applied field. The applied field        is kept below the threshold above which mobility becomes field        dependent. The applied field is also kept below the value above        which a significant proportion of the charge carriers reach the        far side of the diamond and the total charge collected shows        saturation (with blocking contacts; non-blocking contacts can        show gain at this point).    -   (iv) The μτ product is derived by relating the collected charge        to the applied voltage using the Hecht relation.        Q=N ₀ eμτE/D[1−exp{−D/(μτE)}]        -   In this equation Q is the charge collected at the            non-illuminated contact, N₀ the total number of electron            hole pairs generated by the light pulse, E the applied            electric field, D the sample thickness, and μτ is the            mobility and lifetime product to be determined.    -   (v) As an example, if the illuminated electrode is the anode        (cathode), then the charge carriers are generated within a few        μm of the surface, and the charge displacement of the electrons        (holes) to the nearby electrode is negligible. In contrast, the        charge displacement of the holes (electrons) towards the        opposing contact is significant, and limited by the μτ product,        where both μ and τ are specific to the particular charge        carriers moving towards the non-irradiated electrode.

The CVD diamond layer of the invention may be attached to a diamondsubstrate (whether the substrate is synthetic, natural, or CVD diamond).Advantages of this approach include providing a greater overallthickness where the thickness limits the application, or providingsupport for a CVD layer whose thickness has been reduced by processing.In addition, the CVD diamond layer of this invention may form one layerin a multilayer device, where other diamond layers may, for example, bedoped to provide electrical contact or electronic junctions to thediamond layer, or merely be present to provide support for the diamondlayer.

It is important for the production of thick high quality single crystalCVD diamond layers that growth takes place on a diamond surface which issubstantially free of crystal defects. In this context, defectsprimarily means dislocations, other crystal defects and microcracks, butalso includes twin boundaries, point defects, low angle boundaries andany other disruption to the crystal lattice. Preferably the substrate isa low birefringence type Ia or IIb natural, a Ib or IIa highpressure/high temperature synthetic diamond or a CVD synthesised singlecrystal diamond.

The defect density is most easily characterised by optical evaluationafter using a plasma or chemical etch optimised to reveal the defects(referred to as a revealing plasma etch), using for example a briefplasma etch of the type described below. Two types of defects can berevealed:

-   -   1) Those intrinsic to the substrate material quality. In        selected natural diamond the density of these defects can be as        low as 50/mm² with more typical values being 10²/mm², whilst in        others it can be 10⁶/mm² or greater.    -   2) Those resulting from polishing, including dislocation        structures and microcracks in the form of ‘chatter tracks’ along        polishing lines. The density of these can vary considerably over        a sample, with typical values ranging from about 10²/mm², up to        more than 10⁴/mm² in poorly polished regions or samples.

The preferred low density of defects is thus such that the density ofsurface etch features related to defects, as described above, is below5×10³/mm², and more preferably below 10²/mm².

The defect level at and below the substrate surface on which the CVDgrowth takes place may thus be minimised by careful preparation of thesubstrate. Here preparation includes any process applied to the materialfrom mine recovery (in the case of natural diamond) or synthesis (in thecase of synthetic material) as each stage can influence the defectdensity within the material at the plane which will ultimately form thesubstrate surface when processing to form a substrate is complete.Particular processing steps may include conventional diamond processessuch as mechanical sawing, lapping and polishing conditions, and lessconventional techniques such as laser processing or ion implantation andlift off techniques, chemical/mechanical polishing, and both liquid andplasma chemical processing techniques. In addition, the surface R_(A)(arithmetic mean of the absolute deviation of surface profile measuredby stylus profilometer, preferably over 0.08 mm length) should beminimised, typical values prior to any plasma etch being a fewnanometers, i.e. less than 10 nm.

One specific method of minimising the surface damage of the substrate,is to include an in situ plasma etch on the surface on which thehomoepitaxial diamond growth is to occur. In principle this etch neednot be in situ, nor immediately prior to the growth process, but thegreatest benefit is achieved if it is in situ, because it avoids anyrisk of further physical damage or chemical contamination. An in situetch is also generally most convenient when the growth process is alsoplasma based. The plasma etch can use similar conditions to thedeposition or diamond growing process, but with the absence of anycarbon containing source gas and generally at a slightly lowertemperature to give better control of the etch rate. For example, it canconsist of one or more of:

-   -   (i) an oxygen etch using predominantly hydrogen with optionally        a small amount of Ar and a required small amount of O₂. Typical        oxygen etch conditions are pressures of 50-450×10² Pa, an        etching gas containing an oxygen content of 1 to 4 percent, an        argon content of 0 to 30 percent and the balance hydrogen, all        percentages being by volume, with a substrate temperature        600-1100° C. (more typically 800° C.) and a typical duration of        3-60 minutes.    -   (ii) a hydrogen etch which is similar to (i) but where the        oxygen is absent.    -   (iii) alternative methods for the etch not solely based on        argon, hydrogen and oxygen may be used, for example, those        utilising halogens, other inert gases or nitrogen.

Typically the etch consists of an oxygen etch followed by a hydrogenetch and then moving directly into synthesis by the introduction of thecarbon source gas. The etch time/temperature is selected to enable anyremaining surface damage from processing to be removed, and for anysurface contaminants to be removed, but without forming a highlyroughened surface and without etching extensively along extended defects(such as dislocations) which intersect the surface and thus causing deeppits. As the etch is aggressive, it is particularly important for thisstage that the chamber design and material selection for its componentsbe such that no chamber material is transferred by the plasma into thegas phase or to the substrate surface. The hydrogen etch following theoxygen etch is less specific to crystal defects rounding off theangularities caused by the oxygen etch (which aggressively attacks suchdefects) and provides a smoother, better surface for subsequent growth.

The surface or surfaces of the diamond substrate on which the CVDdiamond growth occurs are preferably the {100}, {110}, {113} or {111}surfaces. Due to processing constraints, the actual sample surfaceorientation can differ from these orientations by up to 5°, and in somecases up to 100, although this is less desirable as it adversely affectsreproducibility.

It is also important in the method of the invention that the impuritycontent of the environment in which the CVD growth takes place isproperly controlled. More particularly, the diamond growth must takeplace in the presence of an atmosphere containing substantially nonitrogen, i.e. less than 300 parts per billion (ppb, as a molecularfraction of the total gas volume), and preferably less than 100 partsper billion. The role of nitrogen in the synthesis of CVD diamond,particularly polycrystalline CVD diamond, has been reported in theliterature. For example, it has been noted in these reports that gasphase nitrogen levels of 10 parts per million or greater modify therelative growth rates between the {100} and the {111} faces with anoverall increase in growth rate, and in some cases quality. Further, ithas been suggested that for certain CVD diamond synthesis processes, lownitrogen contents of below a few parts per million may be used. However,none of these reported processes disclose methods of nitrogen analysiswhich are sufficiently sensitive to ensure that the nitrogen content issubstantially below 1 part per million, and in the region of 300 or lessparts per billion. Measurement of nitrogen levels of these low valuesrequires sophisticated monitoring such as that which can be achieved,for example, by gas chromotography. An example of such a method is nowdescribed:

-   -   (1) Standard gas chromatography (GC) art consists of: A gas        sample stream is extracted from the point of interest using a        narrow bore sample line, optimised for maximum flow velocity and        minimum dead volume, and passed through the GC sampling coil        before being passed to waste. The GC sample coil is a section of        tube coiled up with a fixed and known volume (typically 1 cm³        for standard atmospheric pressure injection) which can be        switched from its location in the sample line into the carrier        gas (high purity He) line feeding into the gas chromatography        columns. This places a sample of gas of known volume into the        gas flow entering the column; in the art, this procedure is        called sample injection.        -   The injected sample is carried by the carrier gas through            the first GC column (filled with a molecular sieve optimised            for separation of simple inorganic gases) and is partially            separated, but the high concentration of primary gases (e.g.            H₂, Ar) causes column saturation which makes complete            separation of the nitrogen difficult. The relevant section            of the effluent from the first column is then switched into            the feed of a second column, thereby avoiding the majority            of the other gases being passed into the second column,            avoiding column saturation and enabling complete separation            of the target gas (N₂). This procedure is called            “heart-cutting”.        -   The output flow of the second column is put through a            discharge ionisation detector (DID), which detects the            increase in leakage current through the carrier gas caused            by the presence of the sample. Chemical identity is            determined by the gas residence time which is calibrated            from standard gas mixtures. The response of the DID is            linear over more than 5 orders of magnitude, and is            calibrated by use of special calibrated gas mixtures,            typically in the range of 10-100 ppm, made by gravimetric            analysis and then verified by the supplier. Linearity of the            DID can be verified by careful dilution experiments.    -   (2) This known art of gas chromatography has been further        modified and developed for this application as follows: The        processes being analysed here are typically operating at        70-500×10² Pa. Normal GC operation uses the excess pressure over        atmospheric pressure of the source gas to drive the gas through        the sample line. Here, the sample is driven by attaching a        vacuum pump at the waste end of the line and the sample drawn        through at below atmospheric pressure. However, whilst the gas        is flowing the line impedance can cause significant pressure        drop in the line, affecting calibration and sensitivity.        Consequently, between the sample coil and the vacuum pump is        placed a valve which is shut a short duration before sample        injection in order to enable the pressure at the sample coil to        stabilise and be measured by a pressure gauge. To ensure a        sufficient mass of sample gas is injected, the sample coil        volume is enlarged to about 5 cm³. Dependent on the design of        the sample line, this technique can operate effectively down to        pressures of about 70×10² Pa. Calibration of the GC is dependent        on the mass of sample injected, and the greatest accuracy is        obtained by calibrating the GC using the same sample pressure as        that available from the source under analysis. Very high        standards of vacuum and gas handling practice must be observed        to ensure that the measurements are correct.        -   The point of sampling may be upstream of the synthesis            chamber to characterise the incoming gases, within the            chamber to characterise the chamber environment, or            downstream of the chamber to measure a worst case value of            the nitrogen concentration within the chamber.

The source gas may be any known in the art and will contain acarbon-containing material which dissociates producing radicals or otherreactive species. The gas mixture will also generally contain gasessuitable to provide hydrogen or a halogen in atomic form.

The dissociation of the gas source is preferably carried out usingmicrowave energy in a reactor which may be any known in the art.However, the transfer of any impurities from the reactor should beminimised. A microwave system may be used to ensure that the plasma isplaced away from all surfaces except the substrate surface on whichdiamond growth is to occur and its mount. Examples of preferred mountmaterials are: molybdenum, tungsten, silicon and silicon carbide.Examples of preferred reactor chamber materials are stainless steel,aluminium, copper, gold, platinum.

A high plasma power density should be used, resulting from highmicrowave power (typically 3-60 kW, for substrate diameters of 50-150mm) and high gas pressures (50-500×10² Pa, and preferably 100-450×10²Pa).

Using the preferred conditions described above it has been possible toproduce high quality single crystal CVD diamond layers>2 mm thick (e.g.3.4 mm thick), and to produce from these layers high quality CVDsynthetic cut stones in the form of gemstones, in which three orthogonaldimensions exceed 2 mm (e.g. a round brilliant of 0.31 ct, height 2.6mm, girdle diameter 4.3 mm).

Examples of the invention will now be described.

Example 1

Substrates suitable for synthesising a layer of CVD diamond of theinvention may be prepared as follows:

-   i) Selection of stock material (Ia natural stones and Ib HPHT    stones) was optimised on the basis of microscopic investigation and    birefringence imaging to identify substrates which were free of    strain and imperfections.-   ii) Laser sawing, lapping and polishing processes were used to    minimise subsurface defects using a method of a revealing plasma    etch to determine the defect levels being introduced by the    processing.-   iii) It was possible routinely to produce substrates in which the    density of defects measurable after a revealing etch is dependent    primarily on the material quality and is below 5×10³/mm², and    generally below 10²/mm². Substrates prepared by this process are    then used for the subsequent synthesis.

A high temperature/high pressure synthetic Ib diamond was grown in ahigh pressure press and prepared as a plate using the method describedabove to minimise subsurface defects. The final plate was 5.8 mm×4.9mm×1.6 mm thick, with all faces {100}. The surface roughness at thispoint was less than 1 nm R_(A). This substrate (substrate 1 a) wasmounted, along with a second, similarly prepared, substrate (substrate 1b) on a tungsten substrate using a high temperature braze suitable fordiamond. This was introduced into the reactor and an etch and growthcycle commenced as described above, and more particularly:

-   -   1) The reactor was pre-fitted with point of use purifiers,        reducing nitrogen levels in the incoming gas stream to below 80        ppb, as determined by the modified GC method described above.    -   2) An in situ oxygen plasma etch was performed using 30/150/1200        sccm (standard cubic centimeter per second) of O₂/Ar/H₂ at        237×10² Pa and a substrate temperature of 849° C.    -   3) This moved without interruption into a hydrogen etch at        830° C. with the removal of the O₂ from the gas flow.    -   4) This moved into the growth process by the addition of the        carbon source which in this instance was CH₄ at 30 sccm. The        growth temperature at this stage was 822° C.    -   5) The atmosphere in which the growth took place contained less        than 100 ppb nitrogen, as determined by the modified GC method        described above.    -   6) On completion of the growth period, the two substrates were        removed from the reactor. A CVD layer 3.4 mm thick was released        from substrate (1 a) and prepared as a cut CVD synthetic in the        form of a round brilliant gemstone for experimental purposes        using standard gemstone preparation techniques. This cut        synthetic stone had a height (table to culet) of 2.62 mm and a        weight of 0.31 ct. The CVD layer from substrate (1 b) was used        to prepare a CVD plate for those elements of characterisation        which are difficult to do quantitatively on a round brilliant.    -   7) The CVD synthesised layers were then further characterised by        obtaining the following data provided below and in the attached        FIGS. 1 to 5. (The CVD layers are referred to by the reference        numbers of the substrates on which they were grown.):        -   i) The collection distance of the plate (1 b) was measured            to be >400 μm.        -   ii) The resistivity of plate (1 b) at an applied field of            50V/μm exceeded 1×10¹⁴ Ωcm.        -   iii) The Raman/photoluminescence spectrum of the cut CVD            synthetic stone (1 a) at 77 K, excited using argon ion laser            light at 514 nm was dominated by the Raman line (FIG. 1).            The zero-phonon line at 575 nm was extremely weak and the            ratio of its peak intensity to the Raman peak intensity was            approximately 1:7800.        -   iv) The Raman FWHM line width of the diamond line at 1332            cm⁻¹ for the cut CVD synthetic stone (1 a) was 1.52 cm⁻¹            (measured using 514 nm laser excitation). (FIG. 2).        -   v) The CL spectrum recorded at 77 K for the cut CVD            synthetic stone (1 a) was dominated by extremely strong free            exciton emission at 235 nm. (FIG. 3).        -   vi) The optical absorption spectrum of the optical plate (1            b) showed no extrinsic absorption features and the measured            absorbance at 240 nm was limited only by the reflection            losses expected for diamond (FIG. 4).        -   vii) EPR spectra of the cut CVD synthetic stone (1 a) were            recorded with a Bruker X-band (9.5 GHz) spectrometer at room            temperature. No single substitutional nitrogen could be            detected (P1 EPR centre) with a detection limit of 0.014            ppm. At high powers a weak broad line close to g=2.0028            could be observed, setting an upper limit on the spin            density of 1.6×10¹⁵ cm⁻³ (FIG. 5).

Example 2

The procedure set out in Example 1 was repeated with the followingvariation in conditions:

Two substrates were prepared using the method for low subsurface defectsas described in Example 1. The substrate (2 a) for the cut CVD syntheticstone was 6.8 mm×6.65 mm×0.71 mm thick, with all faces {100}. Again, anadditional similar substrate (2 b), for preparation of an optical plate,was used.

The oxygen etch was at 780° C. for 30 minutes and a net power of 7.8 kW.

The hydrogen etch was at 795° C. for 30 minutes.

Growth occurred with 32 sccm CH₄ added, at a temperature of 840° C.

The atmosphere during growth contained <100 ppb of N₂.

On completion of growth the CVD layer from substrate (2 a) was 2.75 mmthick. This layer was processed as cut CVD synthetic in the form of around brilliant gemstone for experimental purposes using conventionalgemstone processing techniques. The final cut CVD synthetic stone had aweight of 0.3 ct, and had colour and quality grades equivalent to E andVS1 using the standard diamond grading system.

The cut CVD synthetic stone (2 a) and the optical plate (2 b) werefurther characterised by the data provided below and in the attachedfigures.

-   (i) The collection distance of the plate (2 b) was measured to    be >400 μm.-   (ii) The resistivity of plate (2 b) at an applied field of 50 V/μm    exceeded 1×10¹⁴ Ωcm.-   (iii) The Raman/photoluminescence spectrum of the cut CVD synthetic    stone (2 a) at 77 K, excited using argon ion laser light at 514 nm    was dominated by the Raman line (FIG. 6). The zero-phonon line at    575 nm was weak and the ratio of its peak intensity to the Raman    peak intensity was approximately 1:28. The Raman line width (FWHM)    was 1.54 cm⁻¹ (FIG. 7).-   (iv) The CL spectrum recorded at 77 K of the cut CVD synthetic stone    (2 a) was dominated by extremely strong free exciton emission at 235    nm (FIG. 8).-   (v) The optical absorption spectrum of the optical plate (2 b)    showed no extrinsic absorption features and the measured absorbance    was limited only by the reflection losses expected for diamond (FIG.    9).-   (vi) EPR spectra of the cut CVD synthetic stone (2 a) were recorded    with a Bruker X-band (9.5 GHz) spectrometer at room temperature. No    single substitutional nitrogen could be detected (P1 EPR centre)    with a detection limit of 0.014 ppm. At high powers a weak broad    line close to g=2.0028 could be observed, placing an upper limit on    the spin density of 1.6×10¹⁵ cm⁻³ (FIG. 10).

Example 3

A high temperature/high pressure synthetic type Ib diamond was grown ina high pressure press, and prepared using the method described inExample 1 to form a polished plate with low subsurface damage. Thesurface roughness at this stage was less than 1 nm R_(A). The substratewas mounted on a tungsten substrate using a high temperature brazesuitable for diamond. This was introduced into a reactor and a growthcycle commenced as described above, and more particularly:

-   1) The reactor was pre-fitted with point of use purifiers, reducing    nitrogen levels in the incoming gas stream to below 80 ppb, as    determined by the modified GC method described above.-   2) The plasma process was initiated as an oxygen etch using    15/75/600 sccm of O₂/Ar/H₂ at a pressure of 333×10² Pa. This was    followed by a H etch using 75/600 sccm Ar/H₂. Growth was initiated    by the addition of the carbon source which in this instance was CH₄    flowing at 30 sccm The growth temperature at this stage was 780° C.-   3) The atmosphere in which the growth took place contained less than    100 ppb nitrogen, as determined by the modified GC method described    above.-   4) On completion of the growth period, the substrate was removed    from the reactor and the CVD diamond layer, 3.2 mm thick, was    removed from the substrate.-   5) The μτ product measured at 300 K using the Hecht relationship, as    described above, was 3.3×10⁻³ cm²/V and 1.4×10⁻³ cm²/V for electrons    and holes respectively, with an average μτ product of about 2.3×10⁻³    cm²/V.-   6) A space charge limited time of flight experiment measured the    electron mobility, μ_(e), to be 4000 cm²/Vs at a sample temperature    of 300 K.-   7) A space charge limited time of flight experiment measured the    hole mobility, μ_(h), to be 3800 cm²/Vs at a sample temperature of    300 K.-   8) SIMS measurements showed that there is no evidence for any single    impurity present in concentrations above 5×10¹⁶ cm⁻³ (excluding H    and its isotopes).

The measured resistivity was in excess of 2×10¹³ Ohm cm at an appliedvoltage of 50 V/μm as measured at 300 K. The breakdown voltage exceeded100 V/μm.

Example 4

The procedure set out in Example 3 was repeated to produce a furtherdiamond layer. Various properties of this layer (obtained at 300 K) andthe layers of Examples 1 to 3 are set out in the following table:

Thickness Plate As Grown Thickness μ_(e)τ_(e) Example (μm) (μm) CCD (μm)(cm²/V) Ex 1 3 400 420 >400* Ex 2 2 750 435 >400* Ex 3 3 200 500 >480*3.3 × 10⁻³ Ex 4 2 100 280 1.7 × 10⁻³ Resistivity μ_(h)τ_(h) μ_(e) μ_(h)(Ω cm) at 50 Example (cm²/V) (cm²/Vs) (cm²/Vs) V/μm Ex 1 >1 × 10¹⁴ Ex2 >1 × 10¹⁴ Ex 3  1.4 × 10⁻³ 4,000 3,800 >2 × 10¹³ Ex 4 0.72 × 10⁻³*Minimum value, limited by sample thickness

FIGURE CAPTIONS

FIG. 1: Raman/photoluminescence spectrum of cut CVD synthetic stone (1a) recorded at 77 K with 514 nm argon ion laser excitation.

FIG. 2: Room temperature Raman spectrum of cut CVD synthetic stone (1 a)(514 nm argon ion laser excitation) showing a Raman linewidth (FWHM) of1.52 cm⁻¹.

FIG. 3: Cathodoluminescence spectra recorded at 77 K from two locationson the cut CVD synthetic stone (1 a) showing strong 235 nm free excitonemission.

FIG. 4: UV absorption spectrum of optical plate (1 b).

FIG. 5: X-band (9.5 GHz) EPR spectra taken at room temperature on cutCVD synthetic stone (1 a), showing the absence of P1 and the weak broadline close to g=2.0028 which could be observed at high powers. Note thatthe scale of the plot for the sample is 10 000 times larger than that ofthe reference sample.

FIG. 6: Raman/photoluminescence spectrum of cut CVD synthetic stone (2a) recorded at 77 K with 514 nm argon ion laser excitation.

FIG. 7: Room temperature Raman spectrum of cut CVD synthetic stone (2 a)(514 nm argon ion laser excitation) showing a Raman linewidth (FWHM) of1.54 cm⁻¹.

FIG. 8: CL free exciton emission spectrum recorded at 77 K from cut CVDsynthetic stone (2 a).

FIG. 9: UV/visible absorption spectrum of optical plate (2 b).

FIG. 10: Bruker X-band (9.5 GHz) EPR spectra of cut CVD synthetic stone(2 a) taken at room temperature, showing the absence of P1 and the weakbroad line close to g=2.0028 which could be observed at high powers.Note that the scale of the plot for the sample is 10 000 times largerthan that of the reference sample.

1. A layer of single crystal CVD diamond having a thickness of greaterthan 2 mm, wherein the layer has one or more of the followingcharacteristics: (a) a level of any single impurity of not greater than1 ppm and a total impurity content of not greater than 5 ppm whereimpurity excludes hydrogen and its isotopic forms; and (b) in electronparamagnetic resonance (EPR), a single substitutional nitrogen centre[N—C]⁰ at a concentration<100 ppb.
 2. A layer of single crystal CVDdiamond according to claim 1 having a thickness of greater than 2.5 mm.3. A layer of single crystal CVD diamond according to claim 1 having athickness of greater than 3 mm.
 4. A layer of single crystal CVD diamondaccording to claim 1, wherein the layer has both (a) and (b).
 5. A layerof single crystal diamond CVD diamond material according to claim 1,wherein the layer has one or more of the following characteristics: (a)a photoluminescence line related to the (PL) cathodoluminescence (CL)line at 575 nm, measured at 77 K under 514 nm Ar ion laser excitation(nominally 300 mW incident beam), which has a peak height< 1/25 of thediamond Raman peak at 1332 cm⁻¹; (b) a strong free exciton (FE)emission; the strength of free exciton emission excited by 193 nm ArFexcimer laser at room temperature is such that the quantum yield forfree exciton emission is at least 10⁻⁵; and (c) in EPR, a spindensity<1×10¹⁷ cm⁻³ at g=2.0028.
 6. A layer of single crystal diamondCVD diamond material according to claim 4, wherein the layer has one ormore of the following characteristics: (a) a photoluminescence linerelated to the (PL) cathodoluminescence (CL) line at 575 nm, measured at77 K under 514 nm Ar ion laser excitation (nominally 300 mW incidentbeam), which has a peak height< 1/25 of the diamond Raman peak at 1332cm⁻¹; (b) a strong free exciton (FE) emission; the strength of freeexciton emission excited by 193 nm ArF excimer laser at room temperatureis such that the quantum yield for free exciton emission is at least10⁻⁵; and (c) in EPR, a spin density<1×10¹⁷ cm⁻³ at g=2.0028.
 7. A layerof single crystal CVD diamond according to claim 5, wherein the layerhas two or more of said characteristics.
 8. A layer of single crystalCVD diamond according to claim 6, wherein the layer has two or more ofsaid characteristics.
 9. A layer of single crystal CVD diamond accordingto claim 5, wherein the layer has characteristic (a).
 10. A layer ofsingle crystal CVD diamond according to claim 5, wherein the layer hascharacteristic (b).
 11. A layer of single crystal CVD diamond accordingto claim 5, wherein the layer has characteristic (c).
 12. A layer ofsingle crystal CVD diamond according to claim 6, wherein the layer hascharacteristic (a).
 13. A layer of single crystal CVD diamond accordingto claim 6, wherein the layer has characteristic (b).
 14. A layer ofsingle crystal CVD diamond according to claim 6, wherein the layer hascharacteristic (c).
 15. A layer of single crystal CVD diamond accordingto claim 7, wherein the layer has characteristic (a).
 16. A layer ofsingle crystal CVD diamond according to claim 7, wherein the layer hascharacteristic (b).
 17. A layer of single crystal CVD diamond accordingto claim 7, wherein the layer has characteristic (c).
 18. A layer ofsingle crystal CVD diamond according to claim 8, wherein the layer hascharacteristic (a).
 19. A layer of single crystal CVD diamond accordingto claim 8, wherein the layer has characteristic (b).
 20. A layer ofsingle crystal CVD diamond according to claim 8, wherein the layer hascharacteristic (c).
 21. A layer of single crystal CVD diamond accordingto claim 5, having a thickness of greater than 2.5 mm.
 22. A layer ofsingle crystal CVD diamond according to claim 5, having a thickness ofgreater than 3 mm.
 23. A layer of single crystal diamond according toclaim 5, attached, at least in part, to a substrate.
 24. An articlecomprising a layer of single crystal CVD diamond according to claim 1attached, at least in part, to a substrate.
 25. A diamond in the form ofa gemstone produced from a layer of single crystal CVD diamond accordingto claim
 1. 26. A CVD diamond produced in the form of a gemstone from alayer of single crystal CVD diamond according to claim 1 and which hasthree orthogonal dimensions greater than 2 mm, where at least one axislies either along the <100>crystal direction or along the principlesymmetry axis of the stone.
 27. A method of producing a layer of singlecrystal CVD diamond according to claim 1 comprising providing a diamondsubstrate having a surface which is substantially free of crystaldefects, providing a source gas, dissociating the source gas andallowing homoepitaxial diamond growth on the surface which issubstantially free of crystal defects in an atmosphere which containsless than 300 parts per billion nitrogen to produce a layer of singlecrystal CVD diamond having a thickness of greater than 2 mm.
 28. Amethod according to claim 27 wherein the substrate is a lowbirefringence type la or IIb natural, or a lb or lla high pressure/hightemperature synthetic diamond.
 29. A method according to claim 27wherein the substrate is a CVD synthesized single crystal diamond.
 30. Amethod according to claim 27 wherein the surface on which diamond growthoccurs has a density of surface etch features related to defects below5×10³/mm².
 31. A method according to claim 27 wherein the surface onwhich diamond growth occurs has a density of surface etch featuresrelated to defects below 10²/mm².
 32. A method according to claim 27wherein the surface on which the diamond growth occurs is subjected to aplasma etch to minimize surface damage of the surface prior to diamondgrowth.
 33. A method according to claim 32 wherein the plasma etch is anin situ etch.
 34. A method according to claim 32 wherein the plasma etchis an oxygen etch using an etching gas containing hydrogen and oxygen.35. A method according to claim 34, wherein the oxygen etch conditionsare a pressure of 50 to 450×10² Pa, an etching gas containing oxygencontent of 1 to 4%, an argon content of up to 30% and the balancehydrogen, all percentages being by volume, a substrate temperature of600 to 1100° C., and an etch duration of 3 to 60 minutes.
 36. A methodaccording to claim 32, wherein the plasma etch is a hydrogen etch.
 37. Amethod according to claim 36, wherein the hydrogen etch conditions are apressure of 50 to 450×10² Pa, an etching gas containing hydrogen and upto 30% by volume argon, a substrate temperature of 600 to 1100° C., andan etch duration of 3 to 60 minutes.
 38. A method according to claim 32,wherein the surface on which the diamond growth occurs is subjected toboth an oxygen etch and a hydrogen etch to minimize damage to thesurface prior to diamond growth.
 39. A method according to claim 38,wherein the oxygen etch is followed by a hydrogen etch.
 40. A methodaccording to claim 32, wherein the surface R_(A) of the surface on whichthe diamond growth occurs is less than 10 nanometers prior to thatsurface being subjected to the plasma etching.
 41. A method according toclaim 27, wherein the surface on which the diamond growth occurs is a{100}, {110}, {113} or {111} surface.
 42. The method according to claim27, wherein the dissociation of the source gas occurs using microwaveenergy.
 43. The method according to claim 27, wherein said layer has athickness of greater than 2 mm.
 44. An article comprising a layer ofsingle crystal CVD diamond according to claim 5, attached, at least inpart, to a diamond substrate.
 45. A diamond in the form of a gemstoneproduced from a layer of single crystal CVD diamond according to claim5.
 46. A diamond in the form of a gemstone produced from a layer ofsingle crystal CVD diamond according to claim
 6. 47. A CVD diamondproduced in the form of a gemstone from a layer of single crystal CVDdiamond according to claim 5, and having three orthogonal dimensionsgreater than 2 mm, where at least one axis lies along the <100>crystaldirection or along the principle symmetry axis of the stone.
 48. A CVDdiamond according to claim 47, and having three orthogonal dimensionsgreater than 2.5 mm, where at least one axis lies either along the<100>crystal direction or along the principle symmetry axis of thestone.
 49. A CVD diamond according to claim 47, and by having threeorthogonal dimensions greater than 3 mm, where at least one axis lieseither along the <100> crystal direction or along the principle symmetryaxis of the stone.